(In reply to Michel Dänzer from comment #9)
> (In reply to Dennis Schridde from comment #0)
> > During a boot like this, the second card (AMD Radeon RX 560) fails to come up
> > and is not available to the system.
> 
> That's actually because of:
> 
>  [drm:gfx_v8_0_ring_test_ring [amdgpu]] *ERROR* amdgpu: ring 0 test failed
> (scratch(0xC040)=0xCAFEDEAD)
>  [drm:amdgpu_device_init [amdgpu]] *ERROR* hw_init of IP block <gfx_v8_0>
> failed -22
>  amdgpu 0000:01:00.0: amdgpu_init failed
> 
> the other messages are probably mostly harmless / not directly related to
> this problem.
> 
> 
> > * Hangs of the entire system when I start Steam using `env DRI_PRIME=1
> > steam` (nothing reacts to commands anymore, including mouse clicks, the power
> > button and the num-lock key, and the mouse cursor moves very sluggishly)
> 
> That's probably related to the above.

Clarification / more information: The missing RX 560 happens for a few boots
*after* the full system hang.  I.e. first I run Steam with DRI_PRIME=1, click
around for a bit until the system hangs (sometimes waiting alone seems to be
enough, though), then I hard-reset the system, when Linux started the RX 560 is
missing, I reboot, RX 560 is still missing, ... (loop for a few iterations)
..., RX 560 is back and we have a "regular boot".

> > * The firmware and GRUB (and Linux, initially) display at 1024x768, while
> > the monitor's native resolution is 2560x1080.
> 
> That's a motherboard firmware / video card ROM issue, nothing to do with the
> Linux kernel / drivers.

One more bit of information (though probably still unrelated): The resolution
is correct when I do not plug in the RX 560, or connect the display to the RX
560 directly (and setup the mainboard firmware to use the dGPU as primary video
adapter).